Monaco claims first victim Trulli
The Toyota driver, who won on the streets of Monte Carlo with Renault in 2004, had to abandon his broken car after smacking the wall on the approach to the tunnel.
He had done 16 laps with a quarter of an hour of the 90 minute session to run.
Shortly afterwards, Nick Heidfeld was spotted crouching at the rear of his abandoned BMW-Sauber, which spent the dying minutes of the initial practice session mingling with the marshals outside the casino.
The session was disrupted by a 15-minute red-flag period when circuit workers fixed a suspected dislodged drain cover at the exit of turn one.
World championship leader Kimi Raikkonen finished with the fastest time, despite a late brush with the barrier at the entrance to the tunnel.
He was about three tenths of a second ahead of McLaren's Lewis Hamilton and Heikki Kovalainen, and his Ferrari teammate Felipe Massa respectively, all of whom posted laps within a tenth of a second of each other.
The session then ended with drivers allowed to put in a practice start on the actual grid.